Power systems

Abstract

A method of controlling a power system that includes an electrical machine, e.g., wind turbine generator, a power converter, a DC circuit and a dynamic braking system (DBS) having a braking circuit having a braking resistor and being connected in series to the DC circuit, is provided. The method includes operating the DBS and controlling operation of the electrical machine based on a prevailing temperature of the braking circuit, stopping the electrical machine and controlling the electrical machine to be restarted at its rated output power once the prevailing temperature of the braking resistor reaches or falls below a lower temperature threshold. The electrical machine may be restarted at a lower output power and after restarting, its output power can be increased based on a power starting profile as the braking resistor cools.

Claims

1. A method of controlling a power system comprising an electrical machine, a power converter including AC terminals connected to the electrical machine and DC terminals, a DC circuit connected to the DC terminals of the power converter, and a dynamic braking system comprising a braking circuit connected in series to the DC circuit, the braking circuit including a braking resistor and a chopper; the method comprising: in an event of a fault, operating the dynamic braking system for a maximum operation time to dissipate any excess energy in the DC circuit, wherein the maximum operation time is dynamically adjusted as a prevailing temperature of the braking circuit varies; and controlling operation of the electrical machine by stopping operation of the electrical machine until the fault is cleared and restarting the electrical machine dependent upon an acceptable level of the prevailing temperature of the braking circuit.

2. The method according to claim 1, wherein dynamically adjusting of the maximum operation time includes operating the dynamic braking system and comparing the prevailing temperature of the braking circuit in real time against an upper temperature threshold of the braking circuit until the prevailing temperature of the braking circuit reaches or exceeds the upper temperature threshold.

3. The method according to claim 1, wherein the controlling step includes restarting the electrical machine when the prevailing temperature of the braking circuit reaches or falls below a lower temperature threshold.

4. The method according to claim 3, wherein the prevailing temperature of the braking circuit is the prevailing temperature of the braking resistor.

5. The method according to claim 4, wherein the lower temperature threshold is selected to allow the dynamic braking system to dissipate a pre-determined output power of the electrical machine.

6. The method according to claim 5, wherein the pre-determined output power is at least the rated output power of the electrical machine.

7. The method according to claim 1, wherein the controlling step includes restarting the electrical machine after a period of time at a low maximum output power and, after restarting, varying the maximum output power of the electrical machine by increasing the maximum output power as the prevailing temperature of the braking circuit decreases.

8. The method according to claim 7, wherein the maximum output power of the electrical machine is varied as the prevailing temperature of the braking resistor varies wherein the prevailing temperature of the braking resistor is compared in real time against an upper temperature threshold of the braking resistor.

9. The method according to claim 7, wherein the maximum output power of the electrical machine corresponds to a power starting profile determined using a temperature difference of the braking circuit to select a power reference to control the maximum output power for restarting the electrical machine.

10. The method according to claim 9, wherein the power starting profile relates the maximum output power of the electrical machine to a temperature difference between the prevailing temperature of the braking resistor and an upper temperature threshold of the braking resistor.

11. The method according to claim 1, wherein the controlling step includes controlling one or more of the operating parameters of the electrical machine.

12. The method according to claim 11, wherein the electrical machine is operated as a motor during the controlling step.

13. A power system comprising: an electrical machine; a power converter including AC terminals connected to the electrical machine and DC terminals; a DC circuit connected to the DC terminals of the power converter; a dynamic braking system comprising a braking circuit connected in series to the DC circuit, the braking circuit including a braking resistor and a chopper; and a control unit adapted to: in an event of a fault, operate the dynamic braking system for a maximum operation time to dissipate any excess energy in the DC circuit, wherein the maximum operation time is dynamically adjusted as a prevailing temperature of the braking circuit varies; and control operation of the electrical machine by stopping operation of the electrical machine until the fault is cleared and restarting the electrical machine dependent upon an acceptable level of the prevailing temperature of the braking circuit.

14. The power system according to claim 13, wherein the electrical machine is a wind turbine generator whose rotor is driven to rotate by a rotor assembly including one or more rotor blades or a variable speed motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a power system with a wind turbine generator and a DBS having one braking circuit;

(2) FIG. 2 shows a braking circuit;

(3) FIG. 3 shows an alternative power system with a wind turbine generator, multiple channels and a DBS having two braking circuits per channel;

(4) FIG. 4 shows the steps of a method according to the present invention for controlling a wind turbine generator;

(5) FIG. 5 shows an example of how the maximum operation time of the DBS can be determined;

(6) FIG. 6 shows cooling profiles of a braking resistor;

(7) FIG. 7 shows an example of how a power reference for a wind turbine generator can be determined; and

(8) FIG. 8 shows the output power of a wind turbine generator;

(9) FIG. 9 shows a power system with a variable speed motor and a DBS having one braking circuit;

(10) FIG. 10 shows an example of how a speed reference for a variable speed motor can be determined; and

(11) FIG. 11 shows the steps of a method according to the present invention for controlling a variable speed motor.

DETAILED DESCRIPTION

(12) The present invention provides a method of controlling the power systems 1 and 1 shown in FIGS. 1 to 3.

(13) Unless otherwise stated, the following description assumes that the DBS has a single braking circuit. But commentary is provided to explain how the method can be adapted for a DBS with two or more braking circuits.

(14) With reference to FIG. 4, in the event of a fault which prevents power generated by the wind turbine generator 8 from being supplied to the power network or grid 12, e.g., during a low voltage dip, the method includes the following steps:

(15) Step 1: Operating the dynamic braking system (or DBS), e.g., by switching the chopper 24 on and off according to a duty cycle, so that the excess energy in the DC circuit 4 which cannot be supplied to the power network or grid 12 because of the fault is dissipated as heat in the braking resistor 22.

(16) Step 2: Stopping operation of the wind turbine generator 8, e.g., using blade pitch control and/or applying mechanical braking of the blade assembly 10, to bring the wind turbine generator to a complete stop until the fault is cleared. (It will be readily understood that the operations in Steps 1 and 2 will normally overlap and that the stopping of the wind turbine generator 8 can follow a power shedding profile which in practice can be based on a combination of dissipating energy in the DBS and appropriate control to reduce the rotational speed of the wind turbine generator. To this extent, the operations in Step 1 and 2 can be viewed as being part of a single, coordinated, step for controlling the dissipation of excess energy during a fault condition.)

(17) Step 3: Controlling the restarting of the wind turbine generator 8 based on a prevailing temperature of the braking circuit 2.

(18) Maximum Operation Time

(19) In Step 1, the DBS is operated for a period of time (the operation time).

(20) If the DBS is operated for too long, it can result in component failure or, in some cases, the destruction of the DBS or the power system as a whole. In a conventional method, the maximum operation time is a fixed time period that is selected to prevent the components of the DBS from reaching their thermal limits. The fixed time period might be 9 seconds, for example. In the method of the present invention, the maximum operation time can be determined dynamically based on a prevailing temperature of the braking circuit 2, and in particular the prevailing temperature of one or both of the braking resistor 22 and the chopper 24. With reference to FIG. 5, the prevailing temperatures can be estimated accurately using thermal models 30, 32. The thermal model 30 for the chopper 24 uses appropriate inputs 34, e.g., power loss in the chopper, the ambient temperature at the outer case of the chopper, thermal impedance etc., to accurately estimate the junction temperature of the chopper T.sub.c (t) which is then compared in real time against an upper temperature threshold T.sub.c_upper for the chopper. Similarly, the thermal model 32 for the braking resistor 22 uses appropriate inputs 36, e.g., power loss in the braking resistor, the ambient temperature at the resistor etc., to accurately estimate the prevailing temperature of the braking resistor T.sub.r(t) which is then compared in real time with an upper temperature threshold T.sub.r_upper for the braking resistor. Sensors (not shown) for measuring the various temperatures can be provided as part of the power system. Such sensors can provide the ambient temperatures for use in the thermal models 30, 32.

(21) The upper temperature threshold can be the rated temperature for the respective component, i.e., taken from the manufacturer datasheet, or can be selected to be below the rated temperature to provide a particular thermal margin for the respective component. For example, if the rated temperature of the braking resistor is 1200 C., the upper temperature threshold T.sub.r_upper can be selected to be about 1200 C. or can be selected to be less than 1200 C. (e.g., about 1100 C.) so that operation of the DBS is ended before the rated temperature of the braking resistor 22 is reached or exceeded.

(22) When the prevailing temperature T.sub.c(t), T.sub.r(t) of one of the chopper 24 and the braking resistor 22 reaches or exceeds the respective upper temperature threshold, the operation of the DBS in Step 1 is ended. Operation of the DBS can be ended by controlling the control unit 26 to switch off the chopper 24 or prevent further switch on if the chopper is being controlled using a duty cycle.

(23) The maximum operation time t.sub.op for the DBS can be determined by:
t.sub.op=min(t.sub.c_op,t.sub.r_op)
where: t.sub.c_op is the time taken for the prevailing temperature T.sub.c(t) of the chopper 24 to reach the upper temperature threshold T.sub.c_upper, and t.sub.r_op is the time taken for the prevailing temperature T.sub.r(t) of the braking resistor 22 to reach the upper temperature threshold T.sub.r_upper.

(24) If the DBS includes two or more braking circuits, each braking circuit will dissipate a proportion of the output power of the wind turbine generator 8. The operation of the DBS is ended when the prevailing temperature T.sub.c(t), T.sub.r(t) of one of the chopper 24 and the braking resistor 22 in any one of the braking circuits reaches or exceeds the respective upper temperature threshold. In practice, this means that some of the braking circuits might be within acceptable thermal limits when operation of the DBSand hence operation of all of the braking circuitsis ended.

(25) Simulations indicate that for practical implemented wind turbine generators, the maximum operation time determined dynamically based on a prevailing temperature of the braking circuit can be significantly longer than the fixed operation time currently being used. For example, it may be possible to dissipate 3 MW in one braking circuit 2 using a suitable power shedding profile where the DBS is operated for about 15 seconds without the chopper 24 or the braking resistor 22 of the braking circuit reaching or exceeding an respective upper temperature threshold.

(26) If the prevailing temperature of neither the chopper 24 nor the braking resistor 22 of each braking circuit has reached or exceeded the respective upper temperature threshold, the operation of the DBS in Step 1 can be ended when the output power of the wind turbine generator 8 falls to zero or substantially zero and all of the energy has effectively been dissipated.

(27) Minimum Recovery Time

(28) Once the operation of the DBS in Step 1 has ended, the DBS must normally be allowed a period of time to recover so that it is capable of being operated again if necessary. In a conventional method, the minimum recovery time is a fixed time period that is selected to allow the components of the braking circuit, and in particular the braking resistor 22, to cool down sufficiently. The fixed time period might be 20 minutes, for example. During this fixed time period, the wind turbine generator 8 must remain stopped and cannot be used to generate power.

(29) In a first method of the present invention, the wind turbine generator 8 is restarted in Step 3 at its rated output power, i.e., on restarting the wind turbine generator 8 it is ramped up to its rated output power according to a starting sequence. This means that the DBS must be capable of dissipating at least the rated output power of the wind turbine generator 8 and the wind turbine generator cannot be restarted until the braking resistor 22 has cooled sufficiently to provide that capability. It will be readily understood that if the DBS includes two or more braking circuits, each braking circuit must be capable of dissipating its proportion of the rated output power of the wind turbine generator 8.

(30) Instead of using a fixed recovery time, e.g., 20 minutes, the first method of the present invention assumes that the wind turbine generator 8 can be safely restarted when a prevailing temperature T.sub.r(t) of the braking resistor 22 reaches or falls below a lower temperature threshold T.sub.r_lower. The lower temperature threshold T.sub.r_lower is selected so that the DBS is capable of safely dissipating a pre-determined output power of the wind turbine generator 8 at a particular duty cycle. The pre-determined output power is at least the rated output power of the wind turbine generator 8 and can be higher than the rated output power for the worst case situation described above.

(31) The minimum recovery time t.sub.rec for the DBS can be determined by:
t.sub.rec=t.sub.r_rec
where t.sub.r_rec is the time taken for the prevailing temperature T.sub.r(t) of the braking resistor 22 to reach the lower temperature threshold T.sub.r_lower. The prevailing temperature T.sub.r(t) of the braking resistor 22 can be accurately estimated using a thermal model as described above.

(32) FIG. 6 shows simulated cooling profiles of a braking resistor 22 for two different maximum temperatures T.sub.max1 and T.sub.max2 reached during the operation of the DBS in Step 1. The cooling profile will normally depend on the thermal characteristics of the braking resistor (which can be modelled or simulated using an appropriate model) and the ambient temperature of the braking resistor. It can be seen that for the first cooling profile with maximum temperature T.sub.max1, the prevailing temperature T.sub.r1(t) of the braking resistor 22 will reach the lower temperature threshold T.sub.r_lower at time t.sub.r_rec1 and for the second cooling profile with maximum temperature T.sub.max2, the prevailing temperature T.sub.r2(t) of the braking resistor will reach the lower temperature threshold T.sub.r_lower at time t.sub.r_rec2, where T.sub.max2>T.sub.max1 and t.sub.r_rec2>t.sub.r_rec1.

(33) Simulations indicate that for practical implemented wind turbine generators, the minimum recovery time determined dynamically based on a prevailing temperature of the braking circuit, and in particular the braking resistor 22, can be significantly shorter than the fixed recovery time currently being used. In some cases, the DBS might not require any recovery time such that the wind turbine generator 8 can be restarted as soon as the fault in the power network or grid has been cleared and other safety checks have been carried out. In other cases, the minimum recovery time for the DBS determined dynamically based on a prevailing temperature of the braking circuit can be longer than the fixed recovery time. But this avoids a potentially dangerous situation where the wind turbine generator is restarted too soon and where the DBS would not be capable of providing dynamic braking.

(34) If the DBS has two or more braking circuits, the wind turbine generator 8 will normally only be restarted when all of the braking resistors have reached or fallen below their respective lower temperature thresholds.

(35) In a second method, the wind turbine generator 8 is restarted in Step 3 as soon as the fault in the power network or grid has been cleared and other safety checks have been carried out. But instead of restarting the wind turbine generator 8 at its rated output power, i.e., where it is ramped up to its rated output power according to a starting sequence, the wind turbine generator is restarted at an initial, lower, maximum output power that is determined with reference to the prevailing temperature of the braking resistor. The maximum output power of the wind turbine generator 8 is then varied in real time based on the prevailing temperature of the braking resistor so that the maximum output power of the wind turbine generator 8 increases as the braking resistor 22 cools down. This means that the wind turbine generator 8 only needs to be stopped for a short period of timeand in particular, a period of time such as 2-3 minutes that is certainly much shorter than the conventional fixed recovery time and is often shorter than the minimum recovery time used in the first method of the present invention where the wind turbine generator 8 is restarted at its rated output power. At any time after the wind turbine generator 8 has been restarted, the DBS can be operated safely at a particular duty cycle because the braking resistor 22 is able to withstand the maximum temperature increase that would result from dissipating the maximum output power of the wind turbine generator that is determined with reference to the prevailing temperature of the braking resistor. In practice, the actual output power of the wind turbine generator 8 can be less than the maximum output power and will depend on the operating conditions, e.g., wind speed.

(36) With reference to FIG. 7, a temperature difference T(t) is determined by subtracting the prevailing temperature T.sub.r(t) of the braking resistor 22 from an upper temperature threshold T.sub.r_upper for the braking resistor. The upper temperature threshold can be the rated temperature for the respective component, i.e., taken from the manufacturer datasheet, or can be selected to be below the rated temperature to provide a particular thermal margin. The upper temperature threshold can be the same as the upper temperature threshold for determining the maximum operation time of the DBS in Step 1. Alternatively, different upper temperature thresholds can be used.

(37) A power starting profile 38 uses the temperature difference T(t) to select a power reference P.sub.ref which is used to control the maximum power that the wind turbine generator 8 can output after it is has been restarted. The power reference P.sub.ref can be supplied to the wind turbine generator 8 or its controller/regulator (not shown). As the prevailing temperature T.sub.r(t) of the braking resistor 22 decreases, the temperature difference T(t) will increase and the power reference P.sub.ref will increase according to the power starting profile 38 until the wind turbine generator 8 is able to produce the rated output power.

(38) If the DBS has two or more braking circuits, each having a respective temperature difference T(t), the power starting profile 38 can use one of the temperature differences to select the power reference, e.g., the temperature difference that indicates the lowest capability for a braking circuit to withstand the temperature increase that would result from subsequent operation. Each braking circuit must be capable of dissipating its proportion of the maximum output power of the wind turbine generator 8.

(39) The first and second methods are shown graphically in FIG. 8 for a wind turbine generator 8 with rated output power P.sub.rated. A fault occurs on the power network or grid at time t.sub.o and the DBS is operated to dissipate excess power in the DC circuit as the wind turbine generator 8 is brought to a complete stop at time t.sub.1 and the output power is zero. The excess power to be dissipated in the braking resistor 22 is the rated output power P.sub.rated of the wind turbine generator 8. If the DBS has two or more braking circuits, the excess power to be dissipated in each braking resistor would be a proportion of the rated output power P.sub.rated. It is assumed that the operation of the DBS is also ended at time t.sub.1 and that the braking resistor 22 then starts to cool down according to a cooling profilesee FIG. 6.

(40) In the first method described above, the wind turbine generator 8 remains stopped until the prevailing temperature T.sub.r(t) of the braking resistor 22 reaches the lower temperature threshold T.sub.r_lower at time t.sub.3. The time difference between t.sub.1 and t.sub.3 is the minimum recovery time t.sub.r_rec for the braking resistor 22. At time t.sub.3 the wind turbine generator 8 is restarted and is ramped up to its rated output power P.sub.rated according to a starting sequence as indicated by the dashed line 40.

(41) In the second method described above, the wind turbine generator 8 remains stopped until time t.sub.2 which is after the fault on the power network or grid has been cleared. At time t.sub.2 (which might be 2-3 minutes after t.sub.1, for example) the wind turbine generator 8 is restarted and is ramped up to an initial output power P.sub.0 (as indicated by the solid line 42) that is determined by the power reference P.sub.ref and which represents the maximum power that the wind turbine generator 8 can output for the temperature of the braking resistor 22 at time t.sub.2. (FIG. 8 assumes that the wind turbine generator 8 will output the maximum power that it can, i.e., that the prevailing wind conditions allow it to be operated at the limit set by the power reference P.sub.ref.) As the temperature of the braking resistor 22 decreases, the power reference P.sub.ref will increase and the wind turbine generator 8 can output more power to the power network or grid. Eventually, the wind turbine generator 8 will be able to output the rated output power P.sub.rated when the braking resistor 22 has cooled sufficiently that the DBS can be operated safely for a worst case situation. It can be seen that with the second method, the wind turbine generator 8 can be restarted to generate power much sooner, but at a reduced level.

(42) The present invention provides a method of controlling the power system 1 shown in FIG. 9.

(43) With reference to FIG. 10, the method includes the following steps:

(44) Step 1: When the variable speed motor 50 is operating in a generating mode, e.g., during regenerative braking, operating the DBS, e.g., by switching the chopper 24 on and off according to a duty cycle, so that the excess energy in the DC circuit 46 is dissipated as heat in the braking resistor 22.

(45) Step 2: Operating the variable speed motor 50 in a motoring mode and controlling one or more of its operating parameters, e.g., torque, rotational speed, based on a prevailing temperature of the braking circuit 44.

(46) Maximum Operation Time

(47) In Step 1, the DBS is operated for a period of time (the operation time). In a conventional method, the maximum operation time is a fixed time period that is selected to prevent the components of the DBS from reaching their thermal limits. In the method of the present invention, the maximum operation time can be determined dynamically based on a prevailing temperature of the braking circuit 44, and in particular the prevailing temperature of one or both of the braking resistor 22 and the chopper 24. As described above, the maximum operation time t.sub.op for the DBS can be determined by:
t.sub.op=min(t.sub.c_op,t.sub.r_op)
where: t.sub.c_op is the time taken for the prevailing temperature T.sub.c(t) of the chopper 24 to reach the upper temperature threshold T.sub.c_upper, and t.sub.r_op is the time taken for the prevailing temperature T.sub.r(t) of the braking resistor 22 to reach the upper temperature threshold T.sub.r_upper.

(48) Motoring Control

(49) After the DBS has been operated, the variable speed motor 50 can be controlled to operate in a motoring mode. (The motor can also be temporarily stopped and restarted before being operated in a motoring mode, if necessary). While operating in the motoring mode, the input power supplied through the VSD to the variable speed motor 50 can be varied using a control reference such as a speed reference, for example. The input power can be varied in real time based on the prevailing temperature of the braking resistor, e.g., so that the rotational speed of the motor 50 increases as the braking resistor 22 cools down. At any time, the DBS can be operated safely at a particular duty cycle because the braking resistor 22 is able to withstand the maximum temperature increase that would result from dissipating the power generated by the motor during a subsequent generating mode.

(50) The prevailing temperature of the braking resistor can be measured, estimated using a thermal model, or otherwise derived as described in more detail above. The maximum rotational speed of the variable speed motor 50 can be determined using a speed profile that relates rotational speed to temperature or temperature difference.

(51) With reference to FIG. 11, a temperature difference T(t) is determined by subtracting the prevailing temperature T.sub.r(t) of the braking resistor 22 from an upper temperature threshold T.sub.r_upper for the braking resistor. The upper temperature threshold can be the rated temperature for the respective component, i.e., taken from the manufacturer datasheet, or can be selected to be below the rated temperature to provide a particular thermal margin. The upper temperature threshold can be the same as the upper temperature threshold for determining the maximum operation time of the DBS in Step 1. Alternatively, different upper temperature thresholds can be used.

(52) A speed profile 60 uses the temperature difference T(t) to select a speed reference N.sub.ref which is used to control the input power supplied to the variable speed motor 50 through the VSD and hence control the rotational speed of the motor. As the prevailing temperature T.sub.r(t) of the braking resistor 22 decreases, the temperature difference T(t) will increase and the speed reference N.sub.ref will increase according to the speed profile 60 until the motor is able to run at the rated speed or the speed selected by the VSD depending on the operating requirements of the power system.

(53) The speed reference N.sub.ref is provided to the controller 58 for the VSD where it can be used as part of a control strategy to vary the input power to the motor 50 by controlling the semiconductor switches of the power converters 48, 52. Such control strategies are well known to the skilled person and might, for example, use pulse width modulation (PWM) or any other appropriate control. Other control references/profiles can be used to achieve the desired control of the variable speed motor 50 during the subsequent motoring mode based on the prevailing temperature T.sub.r(t) of the braking resistor 22.

(54) If the DBS has two or more braking circuits, each having a respective temperature difference T(t), the speed profile 60 can use one of the temperature differences to select the speed reference, e.g., the temperature difference that indicates the lowest capability for a braking circuit to withstand the temperature increase that would result from subsequent operation. Each braking circuit must be capable of dissipating its proportion of the power generated by the variable speed motor 50 if it is operated in generating mode, e.g., during regenerative braking.